1. Field of the Invention
The present invention relates to a motor drive, and in particular relates to a motor drive having a function for detecting a failure in a dynamic braking circuit.
2. Description of Related Art
Dynamic braking circuits are used, in the event of a power failure or a failure of an amplifier, to short out terminals of a motor through a resistor and quickly stop the motor by consuming rotational energy as heat. Also, there is known a motor drive that determines the presence or absence of a failure in the dynamic braking circuit (for example, Japanese Unexamined Patent Publication (Kokai) No. 2009-165296). The conventional motor drive adopts the following method for determining the presence or absence of a failure. First, a threshold value is determined in accordance with the resistance of the dynamic braking circuit and a direct current voltage. Next, while a command to open a relay of the dynamic braking circuit is outputted, power transistors are turned on for a short time so that the direct current voltage is applied to the dynamic braking circuit. At this time, a value of current flowing out of the power transistor is detected using a current detection means. By comparison between the detected current value and the threshold value, the presence or absence of a failure in the dynamic braking circuit is determined. When the relay of the dynamic braking circuit is welded, the amount of current flowing is equal to or greater than the threshold value, and therefore a welding failure is determined to be present. When the relay of the dynamic braking circuit is not welded, the amount of current flowing does not reach the threshold value, but slowly increases owing to the inductance of the motor, and therefore no welding failure is determined to be present.
FIG. 1 shows a block diagram of the conventional motor drive. A conventional motor drive 1000 includes a motor drive control circuit 1010, a failure determination circuit 1011, a power transistor unit 1003, a current detection circuit 1006, and a dynamic braking circuit 1005. A direct current power supply 1002 rectifies and converts alternating current power from an alternating current power supply (not shown) into direct current power, and outputs the direct current power to the power transistor unit 1003. The power transistor unit 1003 having power transistors A to F constitutes an inverter that converts the rectified direct current power into three-phase alternating current power to drive a motor 1004. The motor 1004 is provided with resistors ru, rv, and rw, and inductances Lu, Lv, and Lw.
The dynamic braking circuit 1005 is connected to wiring between the power transistor unit 1003 and the motor 1004. The dynamic braking circuit 1005 is provided with resistors Ru, Rv, and Rw. To make the resistors consume electric power generated by the motor 1004, switches S1 and S2, i.e. contacts in the relay are closed.
A method for detecting a failure in the conventional dynamic braking circuit, as shown in FIG. 1, is as follows. First, the motor drive control circuit 1010 outputs a dynamic braking circuit control signal to the failure determination circuit 1011. The failure determination circuit 1011 outputs a power transistor control signal to the power transistors A to F of the power transistor unit 1003, to control the actuation and stop of the motor 1004. The current detection circuit 1006 detects a current flowing from the power transistor unit 1003 into the motor 1004. The current detection circuit 1006 performs a digital-to-analog conversion of a detected current value, and outputs the converted current value to the failure determination circuit 1011.
The failure determination circuit 1011 obtains the dynamic braking circuit control signal outputted from the motor drive control circuit 1010 and the current value outputted from the current detection circuit 1006, to detect, from these signals, a failure in the dynamic braking circuit 1005 such as welding of the contact in the relay, a malfunction of the contact, a break in the resistor, or a disconnection of a connection cable.
The failure determination circuit 1011 determines the presence or absence of the failure in the dynamic braking circuit by comparison of a current value I detected by the current detection circuit 1006 with a threshold value ITH. In FIG. 1, if the power transistors A and F are turned on while the other transistors are turned off, the current value I (=Iu) is represented by the following equation.I≈VDC/2L×t+VDC/2R 
Here, VDC represents the voltage of the direct current power supply 1002, 2L (=Lu+Lw) represents the inductance of the motor 1004, t represents time, and 2R (=Ru+Rw) represents the resistance of the dynamic braking circuit 1005. Also, VDC/2L×t represents a current flowing through the motor 1004, and VDC/2R represents a current flowing through the dynamic braking circuit 1005.
Using the above equation, the method for detecting the presence or absence of a failure in the relay of the dynamic braking circuit will be described. When the current I (Iu, Iv, or Iw) flowing through the power transistors A to F is higher than the threshold value, even though a command to disconnect the dynamic braking circuit 1005 from windings Lu, Lv, and Lw of the motor 1004 is issued, the contact of the relay that establishes connection with the dynamic braking circuit 1005 is determined to be welded and broken.
When the presence or absence of a failure (welding) in the relay of the dynamic braking circuit 1005 is detected, a failure detection command is issued by which, while the switches S1 and S2 in the relay of the dynamic braking circuit 1005 are opened, the power transistors are turned on to apply the direct current voltage VDC to the dynamic braking circuit 1005 for a short time Δt.
FIG. 2 shows variations of the current I with time with and without the presence of welding in the relay, when the resistance R of the dynamic braking circuit is low. When the relay of the dynamic braking circuit 1005 is welded, the direct current voltage VDC is applied to the dynamic braking circuit 1005 and therefore the current detection circuit 1006 detects the current I exceeding the threshold value ITH within the time Δt, as shown by a curve A in FIG. 2, so that welding is determined to be present. On the other hand, when the relay of the dynamic braking circuit 1005 is not welded but opened normally, the direct current voltage VDC is applied to the dynamic braking circuit 1005, but as shown by curve B in FIG. 2, the current I does not exceed the threshold value ITH within the time Δt, so that no welding is determined to be present. As described above, when the resistance R of the dynamic braking circuit 1005 is low, the threshold value ITH can be set at a high value, and thus the presence or absence of a welding in the relay of the dynamic braking circuit 1005 is determined normally.
Also as described above, the threshold value ITH depends on the resistance R of the dynamic braking circuit 1005 and the direct current voltage VDC, and has a condition indicated by the following equation (1).VDC/2R>ITH  (1)
When the resistance R of the dynamic braking circuit 1005 is high, the threshold value ITH has to be lowered. FIG. 3 shows variations of the current I with time with and without the presence of welding in the relay, when the resistance R of the dynamic braking circuit 1005 is high. When the relay of the dynamic braking circuit 1005 is welded, the current detection circuit 1006 detects the current I exceeding the threshold value ITH within the time Δt, as shown by curve C in FIG. 3, so that welding is determined to be present. However, when the relay of the dynamic braking circuit 1005 is not welded, if the threshold value ITH is too low, even a current I that slowly increases owing to the inductance of the motor 1004 upon application of the direct current voltage VDC to the motor 1004 by turning on the power transistors will exceed the threshold value ITH within the time Δt as shown by curve D in FIG. 3. Since the threshold value ITH has to be low, welding is determined to be present, though in actual fact there is no welding.